Starling Forces

Describe the essential features of the micro-circulation including fluid exchange (Starling forces) and control mechanisms present in the pre- and post-capillary sphincters

Interstitial fluid is an ultrafiltrate of plasma, with the net filtration pressure determined by the net effect of opposing hydrostatic and oncotic pressures:

These four variables are known as Starling's forces.

Actual fluid movement is (of course) more complicated. Hydrostatic pressure falls along the capillary, and movement of solute and water are affected by other factors. Some of these are described by the:

Reflection coefficient (σ)This describes the fact that a small amount of protein leaks from the capillary, slightly increasing interstitial oncotic pressure and slightly decreasing capillary oncotic pressure. It is dependent on the interstitial protein content, and has a value between 0 and 1.